To "sidestep" is possible but a bit of a pain. There are two options...

1. You can download the song to a PC then transfer it to your phone and set as a ringtone.
2. If you want to avoid the pc step its possible but probably more annoying. You also need root. You have to first pin the song to your device so that its stored locally on the phone. Then you need to navigate to where the music is locally stored.... data/data/com.google.android.music/files/music.... then you need to figure out which file relates to which song. Once you do that you should be able to navigate to it with a 3rd party music player to set it as an ringtone. Its a pain because google renames the song files as numerical file names like 12345.mp3. No one said it was easy....